Such a device is well known in the technical field of drilling and working oil deposits, and is commonly referred to as an “inflatable packer”.
It serves to separate, temporarily, two contiguous portions of the pipe or the well from each other, e.g. in order to carry out investigations or repairs in one of the portions.
The invention relates more particularly to a sealing device of that kind in the form of an inflatable packer that is carried by a support designed to be inserted into the pipe or the well.
Such a device generally comprises:
In its deformable zone:                a tubular and inflatable “sealing membrane” of circular section, made of a material that is leakproof, flexible, and elastic, being deformable radially outwards under the action of the pressure of an internal fluid so as to press hermetically against the “mechanical structure”;        a “mechanical structure” having the function of withstanding the forces exerted by the inflation pressure applied to the sealing membrane, said mechanical structure possibly being fitted with an “anti-extrusion” device serving the function of preventing the sealing membrane from being extruded through the mechanical structure; and        an “outer skin” that is tubular, being made of a material that is flexible and elastic, being radially deformable by the movement of the mechanical structure.        
At its ends:                a “leaktight connection” providing sealing between the sealing membrane and the end;        a “axial mechanical connection” for fastening the mechanical structure to its end and taking up all of the forces having an axial component as created by the inflation pressure; and        a “radial holding ring” holding the mechanical structure radially when it is subjected to the inflation pressure.        
At rest, the diameters of the device and of the packer are less than the diameter of the pipe or the well.
Once the device has been put into position in the desired zone, the packer is generally inflated by pumping in a liquid such as water, a hydrocarbon, and/or the mud present inside the pipe or the well.
The fluid is raised to a high pressure, suitable for causing the membrane to expand together with the mechanical structure, and for pressing the outer skin firmly against the wall of the zone in question in order to plug it hermetically in temporary or definitive manner.
During inflation, the packer expands radially and shortens simultaneously, depending on the shape with which the mechanical structure was made.
Once the investigation and/or repair operations are over, the packer is deflated and the device is withdrawn.
It can naturally be used again subsequently to plug a new zone of the same pipe or the same well, or to be transferred to another site, into a new pipe or a new well in order to perform the same function.
As an indication, in an application to the oil industry, the packer is generally about 1 meter (m) to 4 m long, with an initial (non-inflated) outside diameter lying in a range of 70 millimeters (mm) to 150 mm, approximately, and presenting a wall thickness (membrane, mechanical structure, and outer skin when not inflated) lying in a range 50 mm to 35 mm approximately.
The material constituting the membrane is generally natural or synthetic rubber.
Appropriate inflation of the packer requires a relatively high internal pressure to be used, presenting a value, still by way of an indication, that is usually about 3×107 pascals (Pa) to 4×107 Pa (30 megapascals (MPa) to 40 MPa).
The wall of the membrane is thus exposed during inflation to very high forces that run the risk of damaging it, or even of causing it to explode. The function of the mechanical structure is to withstand the pressure forces mechanically.
The mechanical structure is generally made up either of cables or of metal strips.
The material of the outer skin is generally natural or synthetic rubber.
In the description below, the following definitions are used:                the term “fiber”: an individual cylindrical formation of very small diameter, e.g. of the order of 0.01 mm to 0.02 mm, generally made of synthetic or organic material, and usually grouped together in the form of a yarn or a roving;        the term “yarn”: a long strand of small diameter, e.g. of the order of 0.1 mm to 0.5 mm, made up of fibers that are twisted and spun together;        the term “roving”: a long strand of small section (less than 1 square millimeter (mm2)), made up of parallel fibers taken together; and        the term “cable”: a bundle of steel wires or synthetic threads braided together, having a diameter of more than 0.5 mm.        
At present, there exist two major categories of expandable sealing device that are known.                a) The first type referred to as a “cable” sealing device uses cables made of steel or synthetic material having a diameter lying in the range 1 mm to 4 mm that are wound helically in the wall of the sealing device and that are fastened at their ends by connection means relying on wedging, crimping, or adhesive bonding using epoxy type resins. The cables are enveloped in a flexible and deformable matrix, based on rubber or on silicone, for example.        
In a known embodiment, a pair of concentric reinforcing sheets are provided, each made up of a series of parallel flexible cables wound helically at a long pitch (i.e. as a small angle of inclination relative to the longitudinal axis of the device), the cables of the two sheets sloping at angles of similar value but in opposite directions.
Initially, i.e. before the membrane is inflated, this angle is equal to about 10° to 18°, for example; during inflation of the membrane it increases to reach a final value of about 35° to 40°.
In elaborate devices, at least one similar, third sheet (auxiliary sheet) is provided that is disposed coaxially with the other two, towards the inside of the membrane but made up of cables that are finer, having a diameter lying in the range 0.5 mm to 1 mm, and that are closer to one another than are the cables of the outer (main) sheets.
The function of the auxiliary sheet is to oppose the phenomenon known as extrusion, whereby the material constituting the wall of the membrane is subjected to creep from the inside towards the outside under the action of the very high internal pressure, which runs the risk of forming a hernia passing through the interstices between some of the reinforcing yarns of the main sheets, and leading to the wall rupturing.
The cables constituting the first two sheets, referred to as main sheets, thus have the function of providing the membrane with mechanical strength, while those of the inner, auxiliary sheet constitute an anti-extrusion barrier.
In that kind of device, the density of the cables is practically identical from one end or the other, all along the axis of the packer.                b) The second type referred to as a “strip” sealing device makes use of long steel strips that are disposed parallel to the axis of the sealing device partially overlapping one another like tiles.        
When the sealing device is inflated, and as the orientation of the strips varies, the strips slide relative to one another like the slats of a venetian blind.
The set of strips is in a cylindrical, annular configuration.
This set is interposed between two annular membranes, an inner membrane that provides sealing for the inflation liquid, and an outer membrane that provides sealing against the wall of the well or the casing that lines the well.
In that kind of device, e.g. as described in patent document U.S. Pat. No. 3,604,732, the metal strips perform two functions: they provide mechanical strength, and they constitute an anti-extrusion barrier.
With both types, considerable force is exerted by the pressure in the end zones (in the vicinity of each radial holding ring), in directions perpendicular to the axis of the sealing device, thus making it necessary to use metal end fittings that are thick and that consequently increase the radial size of the sealing device.
Furthermore, the density of the anti-extrusion structure, which is identical all along the device, is found to be weak in the transition zones situated between the above-mentioned end zones and the zone where the outer skin bears against the wall of the pipe or the well; unfortunately, it is in these transition zones that the anti-extrusion system is under the greatest stress, firstly because of the spacing between the cables in this location, and secondly because the mechanical structure is not supported by the wall.
All of those sealing devices need to comply with contradictory constraints imposed by the conditions encountered in wells.
In particular, they must:                a) withstand a plurality of inflation-deflation cycles while maintaining a size that is close to their initial size;        b) be capable of accepting large deformation ratios, possibly as great as 3:1;        c) present small radial size so as to be capable of being laid through constrictions of limited diameter;        d) withstand high differential inflation pressures, of values that reach 30 MPa to 40 MPa; and        e) be capable of being placed in environments that are aggressive, both in terms of temperatures which may exceed 180° C., and in terms of corrosion, since they can be exposed to a variety of fluids (water, oil, and gas in particular).        
Although in widespread use, both of the above-mentioned types of sealing device present their respective drawbacks, which the present invention seeks to resolve.
The first type of device has good memory qualities and can withstand a plurality of inflation and deflation cycles while conserving a size that is close to its initial size. Nevertheless, its performance is limited in terms of withstanding temperature and inflation pressure, firstly because of the limited density of cables used to provide the mechanical function, and secondly because of the presence of gaps between the cables which are supposed to be providing the anti-extrusion function in the zone situated between the metal ends and the wall.
In addition, the dual arrangement of cables enveloped in the flexible matrix presents a size that is relatively large, increasing the radial size of the sealing device before inflation.
The device described in patent document U.S. Pat. No. 5,340,626 proposes a particular arrangement of cables seeking to attenuate those drawbacks.
The cables present winding angles that are different at the ends and in the central portion in order to limit the force exerted on the end members and thus reduce the size of those members. That device solves the problem associated with extrusion by using a special set of short fibers having the purpose of limiting the extrusion of the elastic matrix through the cables that provide the mechanical strength. The device described solves the problem of the leakproof membrane being extruded between the mechanical structure, but only by adding a structure that is made up of short fibers and that is not sufficient in the zone immediately adjacent to the zone bearing against the wall, whenever the expansion, pressure, and temperature stresses become large.
The second type of device, having strips, is better at withstanding pressure and temperature than the first, and its radial size is relatively small.
However, it has very poor ability at returning to its initial size after an inflation-deflation cycle.
Although of interest, neither of those two known types of device can give complete satisfaction in fulfilling the needs of sealing devices that are used in difficult conditions, by satisfying the various constraints mentioned above.